1. Field of the Invention
The present invention relates to an image recording apparatus such as a laser printer or a copying apparatus of the type in which an image is formed on an image carrier through charging, image exposure and development. More particularly, the present invention is concerned with an image recording apparatus such as a laser beam printer or a copying apparatus, wherein the step of transferring a recorded image to a transfer paper sheet is repeated a plurality of times so that superposed images of different colors are formed on the recording paper sheet so as to present a full-color image. The invention also relates to an image recording method to be used for such an image recording apparatus.
2. Description of the Related Art
A full-color image recording apparatus has been known which employs repetition of a step for transferring a recorded image to a recording paper sheet held on a transfer drum, the image having been formed on an image carrier through electrostatic charging, image exposure and development, so that superposed images of different colors are formed on the recording paper sheet so as to provide a full color image. This type of image recording apparatus is disclosed in, for example, Japanese Unexamined Patent Publication No. 50-50935.
In operation, a position signal 35 is produced when the transfer drum is at a predetermined rotational phase or position, and this signal is used as a vertical synchronizing signal which gives a cue to the start of each image forming cycle for forming an image of each color. Meanwhile, a BD signal 32 is produced when a deflected laser beam passes a predetermined position. This BD signal 32 is used as a horizontal synchronizing signal.
FIG. 13 illustrates general constructions of a full-color image forming apparatus (denoted by 33) and a controller (denoted by 34) which produces image data.
Referring to this Figure, a position signal 35 indicates the leading end of an image as viewed in the direction of a sub-scan. A BD signal 36 indicates the start position of the image in the direction of a main scan. Numeral 37 denotes an image signal. The controller 34 produces image data of each color image component in synchronization with the position signal 35 in the direction of the sub-scan and in synchronization with the BD signal 36 in the direction of the main scan, thus delivering one-line of image data.
As will be detailed later, image data 37-1, 37-2, 37-3 and 37-4 corresponding to magenta, cyan, yellow and black color components are delivered in accordance with the timing chart shown in FIG. 15, whereby a full-color image is printed by the image forming apparatus 33.
A description will now be given of the method of generating the BD signal 32, as well as of the method of generating the position signal 35 by an interrupter, with specific reference to FIG. 14.
FIG. 14 shows, by way of example, blocks of a circuit which pertains to the BD signal 32 and the position signal 35, while FIG. 15 is a timing chart showing timings of various signals appearing in the circuit shown in FIG. 14.
Position signal 35 is supplied to a D flip-flop 38. The output of the D flip-flop 38 is set high when the BD signal 32 rises, so that an AND gate 39 is opened to pass the BD signal 32.
A counter 40 counts BD signals 36 to determine the timing of an output of one-page of image data of each color image component. When the counter 40 has counted up a predetermined number of BD signals 36, a ripple carry of the counter 40 resets the D flip-flop 38. Timings of signals produced in this circuit are shown in FIG. 15. Numerals 41 and 42 denote inverters.
A relationship between the BD signal 32 and the BD signal 36 will be clearly seen from FIG. 15. Image data 37-1, 37-2, 37-3 and 37-4 of respective color image components are produced at timings as shown in FIG. 15, so that a full color image composed of image components of the four colors is printed. FIG. 16 shows, by way of example, the positional relationship between two color image components out of the four color image components. Thus, FIG. 16 illustrates an example of a sample of a print formed in two colors, specifically cyan and yellow, by an image forming apparatus of the kind described.
It will be seen that the cyan image component 36-1 and the yellow image component 36-3 are mis-registered with or staggered from each other by an amount approximating the pitch of the main scan lines.
This mis-registration is attributed to the following reason. It will be seen from the enlarged view (a) in FIG. 15 that the cyan color component 36-1 is formed at the correct position since the timing of rise of the BD signal 32 has happened to coincide with the timing of fall of the position signal 35, whereas, in the time section shown in the enlarged view (b) in FIG. 15, the timing of rise of the BD signal 32 fails to coincide with the timing of fall of the position signal 35, leaving a time difference .DELTA.T therebetween. This time difference .DELTA.T appears as the mis-registration between the two color image components shown in FIG. 16.
A method is proposed in, for example, Japanese Patent Laid-Open No. 4-212563, for eliminating generation of such a time difference .DELTA.T in an image recording apparatus which operates in an asynchronous manner as described above.
The proposed method calculates the time difference between the BD signal 32 and the position signal 35 which is used as a reference, and, if the time difference is greater than half the signal period of the BD signal 32, the BD signal immediately preceding the calculated BD signal is sent to the controller as the BD signal 36, whereas, if not, the calculated BD signal is sent to the controller. According to this method, the amount of mis-registration cannot exceed 1/2 the width of the sub-scan, that is, the mis-registration is suppressed.
It has also been proposed to control a high-speed motor 12 in accordance with the calculated time difference.
The first-mentioned proposed method, however, cannot completely eliminate color mis-registration. The second-mentioned proposed method also is disadvantageous in that, once the speed of the high-speed motor is changed, a considerably long time is required to restore the rotation speed, falling to provide highly accurate and delicate control of the image positions.